La Niña episodes also feature large-scale changes in the atmospheric winds across the
tropical Pacific, including increased easterly (east- to- west) winds across the eastern
Pacific in the lower atmosphere, and increased westerly (west-to-east) winds over the
eastern tropical Pacific in the upper atmosphere. These conditions reflect an enhanced
strength of the equatorial Walker Circulation.

During the developing phase of La Niña, the subsurface ocean structure is
characterized by an abnormally shallow layer of warm water across the eastern tropical
Pacific, in association with a reduced depth of the oceanic thermocline. Thus, the slope
of the thermocline is increased across the basin. In very strong La Niña episodes, the
thermocline can actually come very close to the ocean surface for extended periods. This
cooling represents a decreased depth of the oceanic mixed layer, and accompanies the
appearance of nutrient-rich waters very close to the ocean surface. Thus, its existence is
quite beneficial to marine life throughout the region. Accompanying these conditions, the
sea level height is lower than normal over the eastern Pacific, resulting in an increased
slope of the ocean surface across the basin. There is also considerable evolution in the
subsurface temperature and thermocline structure during both El Niño and La Niña
episodes.